This invention relates to a water heater using light oil, heavy oil and liquid fuel like LNG and LPG as fuel, particularly a multistep water heater which has high combustion efficiency and improved space for fuel combustion and heat transfer surface in the water heater so that the harmful component in the exhaustion gas is reduced and the heat transfer efficiency is increased.
Generally, in most conventional water heaters such as a boiler it cannot be expected that the fuel be completely combusted. Since the conventional water heater has a small heat transfer surface, a small heat transfer region and a short heat transfer time, it also cannot be expected that the high temperature of the combustion gas from the water heater can be fully used. Therefore the high temperature of 200.degree. C.-350.degree. C. is directly exhausted to the atmosphere and the loss of the heat is high.
Furthermore, as the boiler using the lighter oil, heavy oil or gas as fuel is increased, the problems of environment sanitation caused by the harmful component contained in the exhaustion gas are increased.